1. Field of the Invention
The present invention relates to discharging machines, and more particularly, to a servo control method and apparatus for use on discharging machines to control the discharging of electric arc on a working piece.
2. Description of Prior Art
FIG. 1 shows a schematic block diagram of a conventional servo control system for discharging machine, in which the numeral 1 designates circuit representation of the discharging machine, and 2 designates a servo control circuit used to control the feedrate to the discharging machine. The servo control circuit is composed of a voltage divider 3, an averaging circuit 4, and a CNC circuit 5. The voltage divider 3 is used to sample a voltage signal from the voltage between the discharging electrode and the working piece and feed it to the averaging circuit 4 to be averaged there. The averaged signal is then sent to the CNC circuit 5 to be used to control the feedrate of the discharging machine. In this conventional method, the discharge signal has a waveform the same as the output signal 6 of the voltage divider 3, which is illustrated in FIG. 2. The average voltage is obtained in accordance with the following equation: EQU V.sub.g2 =(V.sub.i *T.sub.i +V.sub.d *T.sub.on)/(T.sub.i +T.sub.on +T.sub.off)
wherein
V.sub.g2 is the average voltage,
V.sub.i is the ignition voltage,
V.sub.d is the discharging voltage,
T.sub.i is the ignition delay,
T.sub.on is the length of the duration the discharging is on, and
T.sub.off is the length of the duration the discharging is off.
It can be learned from the foregoing equation that when the average voltage of the discharge is related to the off-time T.sub.off, i.e., a change in T.sub.off, will change the average voltage and thereby affect, the stability of the machining process. Under this circumstance, other settings of the discharging machine must be adjusted in order to restore stability. Further drawbacks of this conventional method include a slow response of the discharge and accumulation of carbon when the machining is insufficient.